Fracturing mechanism of bedding shale under directional fracture-controlled blasting
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摘要: 为探究定向断裂控制爆破下层理页岩的爆破致裂机理，采用切缝药包，对四种切缝角度下的页岩立方体试件进行爆破试验，采用数字图像相关技术（DIC）对页岩试件表面应变场的演化过程进行监测，分析了微裂纹孕育至宏观裂纹贯通的内在机理，并基于盒维数理论计算了不同切缝角度下页岩试件表面裂纹的分形维数，采用Matlab软件对爆后块度的筛分方法进行了编程分析，开发了全自动的粒径分析程序，实现了粒径圈定的可视化。试验结果表明：试件在不同比例爆距内的裂纹总密度与比例爆距之间存在负相关的幂函数关系，切缝方向与层理弱面的夹角对微观损伤区域出现的位置影响显著，当层理弱面与切缝方向平行时，损伤区域多集中于层理弱面处，对宏观裂纹的扩展路径影响显著，易于形成单一裂纹；层理弱面处的能量泄漏是造成页岩爆破破碎效果较差的重要因素，当切缝方向与层理弱面一致时，试件爆后的大块占比较高，爆后块度的分形维数平均值在各组间最低，仅为0.7843，而当切缝方向与层理面垂直时，试件的爆后块度分布较为均匀，爆后块度的分形维数平均值达到了2.5233，爆破破碎效果相对较好。Abstract: The precise control of explosive energy to form an effective radial fracture network in shale is the key of shale gas dynamic extraction. In order to elucidate the damage and fracture mechanisms of shale under directional fracture-controlled blasting and establish a quantifiable relationship for shale damage and destruction under various blasting conditions, explosive tests were conducted on cubic shale specimens with four different fracture angles. The evolution of surface strain fields on the shale specimens was monitored using digital image correlation (DIC) technology. Additionally, the fractal dimensions of surface cracks on the shale specimens at different fracture angles were computed based on the box-counting theory. A programmed analysis of post-blast fragment size distribution was carried out using Matlab software, resulting in the development of a fully automated particle size analysis program with visual delineation of particle sizes. The experimental results demonstrate a negative power-law relationship between crack density and scaled distance within different scaled distances. The angle between the fracture direction and the weak plane of the bedding significantly influences the location of micro-damaged areas. Particularly, when the weak plane of the bedding is parallel to the fracture direction, damaged areas tend to concentrate along the weak plane, affecting the macrocrack propagation path and favoring the formation of a single crack. Energy dissipation at the weak planes of the bedding is identified as a crucial factor leading to suboptimal fracturing effects in shale blasting. When the fracture direction aligns with the weak plane of the bedding, a higher proportion of large fragments is observed in the post-blast specimens. The average fractal dimension of fragment size distribution is the lowest among all groups, measuring only 0.784 3. Conversely, when the fracture direction is perpendicular to the weak plane of the bedding, the distribution of post-blast fragment sizes becomes more uniform. The average fractal dimension of fragment size distribution increases to 2.5233, indicating relatively better blasting fragmentation results in such scenarios.
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图 3 不同切缝方向下典型试件的表面裂纹分布
Figure 3. Surface pattern distribution of typical specimens with different cutting directions
图 5 裂纹密度随比例爆距的变化曲线
Figure 5. Variation curve of crack density with proportional explosion distance
图 6 不同比例爆距的区域裂纹密度变化
Figure 6. Crack density changes in the explosion zone with different proportions
图 7 各切缝角度下典型试件的盒维数拟合曲线
Figure 7. Box dimension fitting curves of typical specimens at various slit angles
图 8 试件B-0纵向应变场演化过程
Figure 8. Longitudinal strain field evolution process of typical specimens of group B-0
图 9 试件B-90应变场演化过程（εxx）
Figure 9. Evolution process of strain field (εxx) of typical specimens of group B-90
图 11 试件B-90应变场演化过程（εyy）
Figure 11. Evolution process of strain field (εyy) of typical specimens of group B-90
图 10 试件B-90应变场演化过程（εxy）
Figure 10. Evolution process of strain field (εxy) of typical specimens of group B-90
图 12 试件B-90宏观裂纹的扩展过程
Figure 12. Macroscopic crack expansion process of typical specimens of B-90 group shale
图 14 试件B-C0块度分布特征
Figure 14. Fragment size distribution characteristics of specimens B-C0
图 15 试件B-0块度分布特征
Figure 15. Fragment size distribution characteristics of specimens B-0
图 16 试件B-45块度分布特征
Figure 16. Fragment size distribution characteristics of specimens B-45
图 17 试件B-90块度分布特征
Figure 17. Fragment size distribution characteristics of specimens B-90
图 18 B-C0试件沿水平层理面的破坏
Figure 18. Failure of B-C0 specimen along horizontal bedding plane
图 19 B-0试件沿水平层理面的破坏
Figure 19. Destruction of B-0 specimen along horizontal bedding plane
图 24 各组试件的爆后碎块分形维数
Figure 24. Fractal dimension of post-blast fragments for each group of specimens
表 1 页岩基础物理力学参数
Table 1. Physical and mechanical parameters of shale
层理倾角/(°) 纵波波速/(m·s−1) 密度/(g·cm−3) 弹性模量/GPa 单轴抗压强度/MPa 单轴抗拉强度/MPa 0 3050.93 2.53 11.604 130.44 3.189 30 3100.53 2.48 11.720 104.41 3.753 60 3184.79 2.57 11.331 93.40 4.475 90 3212.55 2.55 12.373 114.18 4.561 
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表 2 各组试件的块度分布指标
Table 2. Block size distribution index of orthogonal test
试件 d10/mm d50/mm d90/mm dmax/mm CU CC B-C0-1 48.09 88.00 102.20 113.91 1.16 1.58 B-C0-2 41.27 74.14 94.00 118.44 1.27 1.42 B-0-1 23.58 51.79 78.72 85.62 1.52 1.45 B-0-2 21.98 38.01 44.35 63.22 1.17 1.48 B-45-1 19.88 53.60 88.89 98.08 1.66 1.63 B-45-2 21.83 41.53 73.35 75.91 1.77 1.08 B-90-1 31.70 49.03 72.99 82.09 1.49 1.04 B-90-2 30.31 52.27 73.70 88.42 1.41 1.22
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